The reference point in an electrical circuit to measure voltages is called <em>Earth</em> or <em>Ground</em>; that point is a direct hookup to earth that helps stabilizing electrical currents to prevent high voltage surges.
What makes a substance easily grounded is to be a good energy conductor; metallic objects are generally good energy conductors aka GEM <em>"Ground Enhancement Materials"</em> and they are mostly produced from rocks and minerals. That's why the answer is (B.) <em>Iron Nail</em>.
